1. Field of the Invention
This invention relates to a process for producing PZT films by a chemical vapor deposition method.
2. Description of the Related Art
A chemical vapor deposition method (referred to hereinafter as CVD method) is used as a method of efficiently mass-producing PZT films used in highly integrated nonvolatile memories. As the method of feeding their starting materials, there is a system of feeding vapors of Pb, Ti and Zr compounds themselves or a system of feeding the vapors from solutions.
In place of highly volatile, highly toxic tetraethyl lead, lead bis(β-diketonate) particularly lead bis(dipivaloylmethanate) (referred to hereinafter as Pb(dpm)2) has been examined as the Pb source. However, the vapor pressure of this compound is as low as 0.2 Torr at 150° C. Accordingly, the system of feeding a vapor thereof from a solution prepared by dissolving the compound in an organic solvent is advantageous to mass-production of PZT films over the system of feeding a vapor of the compound itself because a larger amount of Pb source can be fed.
THF, octane or butyl acetate is used as the solvent. In vaporization of the solution, it is desired that the respective solutions of Pb, Zr and Ti materials are mixed just before a vaporizer and fed after vaporization in the same vaporizer, or one solution containing Pb, Zr and Ti is used as the starting material and fed after vaporization in the same vaporizer. In the latter system in particular, the facilities used would be simplified and the compositions of the resultant films could be varied less than in the former. In either system, the starting materials are in the form of a mixed solution in the vaporizer at about 200° C. and for about 1 to 10 seconds just before the vaporizer. At this stage, Ti(OiPr)4 and Zr(OtBu)4 whose vaporization characteristics are changed upon reaction with Pb(dpm)2 are not preferable. P. C. van Buskirk et al. disclose, in Integrated Ferroelectrics, Vol. 21, 273 (1998), that a combination of Pb(dpm)2-Zr(dpm)4-Ti(OiPr)2(dpm)2 is preferable.
Further, the temperature at which ZrO2 is deposited from Zr(dpm)4 is considerably higher than the temperature at which PbO is deposited from Pb(dpm)2, so that as Zr compounds showing a lower deposition temperature, Zr(OiPr)2(dpm)2 and Zr2(OiPr)6(dpm)2 are disclosed in WO 98/51837, and Zr(OiPr)(dpm)3 is disclosed in Japanese Patent Application Laid-Open No. 2001-151782. As a Ti source used in combination therewith, however, Ti(OiPr)2(dpm)2 only has been disclosed.
If nonvolatile memories of PZT were produced at temperatures lower than the temperature at which Al circuits are durable, semiconductor devices highly integrated with multi-layered metals could be produced, and thus production of PZT films at a substrate temperature of 500° C. or less, preferably 450° C. or less has been examined (JP-A No. 2000-58526 etc.). Given a combination of Pb(dpm)2-Zr(OtBu)4-Ti(OiPr)4-NO2, PZT films can be produced at this low temperature, but there is no related art teaching that the starting materials are fed in the solution vaporization system and PZT films are produced at temperatures lower than a substrate temperature of 450° C. or less.